I haven't done it personally, but I know Layne Norton has coached a few people who have gotten leaner around 600g carbs/day. That's after months of reverse dieting, though. I would assume that if you were to go from 300-400 carbs to 800+, you would definitely put on body fat. That's 3200 calories from carbs alone! If you're eating 200g protein along with it, that's already 4,000 calories with no fats, which is almost impossible, and definitely unhealthy. If you don't mind putting on some BF then I suppose it would work pretty well, although I know more is not always better, and I don't know what happens if a person eats 800g/carbs for extended lengths of time. I'm interested to hear if anyone on the forums has an idea of what that would do to a body in the short vs long term.